Method and circuit arrangement for detecting the level of a liquid

ABSTRACT

A method for detecting the level of a liquid in a container. The method includes applying a predetermined voltage to a level measuring electrode, disposed within the container, for a predetermined first period of time; then, discontinuing the application of the voltage and waiting a predetermined second period of time; and then measuring an electrode voltage at the level measuring electrode.

TECHNICAL FIELD

This disclosure relates to methods and apparatuses for detecting thelevel of a liquid in a container.

BACKGROUND

Liquid level detection devices are known in the art. For example, U.S.Pat. No. 5,775,164 discloses a device for detecting the level of aliquid in a container. This particular device comprises two electrodesthat are arranged in the container and an electronic circuit that isconnected to the electrodes. In order to prevent measuring voltagesresulting from electrolyzing the liquid, an a.c. voltage is onlygenerated between the electrodes during brief measuring phases in orderto measure the electric resistance between the electrodes.

SUMMARY

According to one aspect, a method for detecting the level of a liquid ina container includes applying a predetermined voltage to a levelmeasuring electrode, disposed within the container, for a predeterminedfirst period of time; then, discontinuing the application of the voltageand waiting a predetermined second period of time; and then measuring anelectrode voltage at the level measuring electrode.

The method also provides the advantage that the level measurement causesreduced (e.g., substantially no) electrolyzing of the liquid, the levelof which can be measured, because the measuring voltage used for themeasurement may consist of a sufficiently short voltage pulse.

In order to continuously monitor the level of a liquid in a container,the level measurement naturally can be repeated periodically. Therefore,in some embodiments, the steps of applying the predetermined voltage,discontinuing the application of the voltage, and measuring theelectrode voltage are repeated after a predetermined third period haselapsed after measuring the electrode voltage. After measuring theelectrode voltage, the level measuring electrode can be connected to anelectric potential, for example, the ground potential, for apredetermined fourth period. By connecting the level measuring electrodeto the electric potential the liquid is prevented from being chargedover time, and therefore unreliable measurements can be prevented.

A suitable circuit arrangement for implementing the method can beprovided in the form of a measuring circuit that, in addition to aground terminal, requires only one terminal for the level measurement,namely because the individual steps are carried out successively. Thiscan be particularly advantageous in instances in which such a measuringcircuit is implemented in a microcontroller or an application-specificintegrated circuit (ASIC) because the number of terminals on amicrocontroller or ASIC represents a significant cost factor. However,it is advantageous to be able to alternately use this terminal as anoutput and an input.

The details of one or more embodiments of the invention are set forth inthe accompanying drawing and the description below. Other features andadvantages of the invention will be apparent from the description anddrawing, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a container liquid level detector.

DETAILED DESCRIPTION

The circuit arrangement shown in FIG. 1 includes a measuring circuit 1provided in a microcontroller and an integrator circuit 3 that includesa resistor 4 and a capacitor 5. Although the microcontroller hasmultiple terminals, only one of these terminals is provided for themeasuring circuit in addition to a ground terminal. This sole terminal 2is reversible, i.e., it can be alternately used as an output and aninput. The integrator circuit 3 is connected to this terminal 2, i.e.,one end of the resistor 4 is connected to the terminal 2 and the otherend of the resistor 4 is connected to a level measuring electrode 8 of aliquid container 7 and to the ground via a capacitor 5. The container 7is also provided with a second electrode 9 that is arranged underneaththe level measuring electrode 8 and connected to the ground.

The measuring sequence is described below. First, the measuring circuit1 delivers a brief voltage pulse at its terminal 2 that is switched intothe output mode. The intensity of this voltage pulse is chosen such thatthe capacitor 5 of the integrator circuit 3 can be almost completelycharged. The two electrodes 8, 9 of the container 7 lie parallel to thecapacitor 5 and also represent a capacitor, the capacitance of which isdependent, however, on whether only the level measuring electrode 8 orboth electrodes 8, 9 are immersed in the liquid 6. Next, the terminal 2of the measuring circuit 1 is reversed during a brief second period andthen represents a high-resistance input of the measuring circuit 1.Leaving aside negligible leakage currents, the capacitor 5 of theintegrator circuit 3 therefore cannot discharge if only the secondelectrode 9 or neither electrode is immersed in the liquid 6. This meansthat the capacitor holds a voltage that approximately corresponds to theamplitude of the voltage pulse received by the capacitor in the firststep. However, if both electrodes 8, 9 are immersed in the liquid, thecapacitor 5 of the integrator circuit 3 is charged to a comparativelylow voltage during the first step because the capacitance of the liquid6 lies parallel to the capacitor in this arrangement. Furthermore, thecapacitor would be able to discharge via the liquid 6 during the secondstep. Then, the measuring circuit 1 detects or measures the voltage atthe capacitor 5 of the integrator circuit 3 during a third period.

The length of the first and the second period and the ratings of theintegrator circuit 3 can be chosen such that the measuring circuit 1merely needs to detect either a HIGH-level or a LOW-level, i.e., thevoltage is not precisely measured in the third step, but rather merelydetected by the microcontroller. This means, in particular, that theresistance of the resistor 4 of the integrator circuit 3 needs to bemuch higher than the resistance of the liquid 6 between the twoelectrodes 8, 9, and that the capacitance of the capacitor 5 of theintegrator circuit 3 needs to be much lower than the capacitance of theliquid between the two electrodes. The measuring cycle including thesteps of applying the predetermined voltage, discontinuing theapplication of the voltage, and detecting the electrode voltage ispreferably not repeated until a fourth period has elapsed afterdetecting the electrode voltage. The fourth period is generally muchlonger than the second and third periods.

1-8. (canceled)
 9. A method of detecting a liquid level in a container,comprising: applying a predetermined voltage to a level measuringelectrode, disposed within the container, for a predetermined firstperiod of time; discontinuing the application of the voltage and waitinga predetermined second period of time; and then measuring an electrodevoltage at the level measuring electrode.
 10. The method according toclaim 9, further comprising waiting a predetermined third period of timeafter measuring the electrode voltage, and then repeating the steps ofapplying the predetermined voltage, discontinuing the application of thevoltage, and measuring the electrode voltage.
 11. The method accordingto claim 9, further comprising connecting the level measuring electrodeto an electric potential following the measuring of the electrodevoltage, thereby preventing electrolyzing of the liquid.
 12. The methodaccording to claim 11, wherein the electric potential is a groundpotential.
 13. The method according to claim 9, further comprisingproviding an integrator circuit that applies the voltage to the levelmeasuring electrode.
 14. The method according to claim 13, wherein theintegrator circuit includes a capacitor, and wherein measuring theelectrode voltage comprises measuring voltage at the capacitor.
 15. Acontainer liquid level detector, comprising: a level measuring electrodedisposed within a container at a first liquid level; a measuring circuitincluding a first terminal, wherein the measuring circuit is configuredto switch the first terminal between an output mode and an input mode;and an integrator circuit that provides an electrical connection betweenthe first terminal of the measuring circuit and the level measuringelectrode.
 16. The container liquid level detector according to claim15, wherein the measuring circuit is configured to apply a supplyvoltage to the level measuring electrode when the first terminal isswitched in its output mode.
 17. The container liquid level detectoraccording to claim 15, wherein the measuring circuit is disposed withina microcontroller.
 18. The container liquid level detector according toclaim 15, further comprising a ground electrode disposed within thecontainer at a second liquid level below the first liquid level.
 19. Thecontainer liquid level detector according to claim 18, wherein themeasuring circuit is configured to detect a measuring voltage across thelevel measuring and ground electrodes when the first terminal is in itsinput mode.
 20. The container liquid level detector according to claim18, wherein the integrator circuit comprises: a resistor having a firstend connected to the first terminal of the measuring circuit, and asecond end connected to the level measuring electrode; and a capacitorhaving a first end connected to the level measuring electrode and secondend of the resistor, and a second end connected to the ground electrodeand a ground terminal of the measuring circuit.
 21. The container liquidlevel detector according to claim 20, wherein the capacitor has acapacitance less than a capacitance of the fluid between the electrodesdetermined when the level of the liquid is at least equal to the firstliquid level.
 22. The container liquid level detector according to claim20, wherein the resistor has a resistance greater than a resistance ofthe fluid between the electrodes determined when the level of the liquidis at least equal to the first liquid level.
 23. The container liquidlevel detector according to claim 20, wherein the measuring circuit isconfigured to detect a measuring voltage across the capacitor.
 24. Amethod of detecting a liquid level in a container, comprising: providinga container liquid level detector, comprising: a level measuringelectrode disposed within the container at a first liquid level, aground electrode disposed within the container at a second liquid levelbelow the first liquid level, a measuring circuit including a firstterminal, wherein the measuring circuit is configured to switch thefirst terminal between an output mode and an input mode, and anintegrator circuit providing an electrical connection between the firstterminal of the measuring circuit and the level measuring electrode;applying a predetermined voltage from the measuring circuit to the levelmeasuring electrode for a predetermined first period of time;discontinuing the application of the voltage and waiting a predeterminedsecond period of time; and then detecting a measuring voltage at thelevel measuring electrode with the measuring circuit after apredetermined second period of time.
 25. The method according to claim24, wherein applying the predetermined voltage includes setting thefirst terminal of the measuring circuit to the output mode anddelivering the predetermined voltage to the level measuring electrodethrough the integrator device.
 26. The method according to claim 24wherein detecting the measuring voltage includes switching the firstterminal of the measuring circuit to the input mode and detecting avoltage across the level measuring and ground electrodes.
 27. The methodaccording to claim 24, wherein the integrator circuit comprises: aresistor having a first end connected to the first terminal of themeasuring circuit, and a second end connected to the level measuringelectrode; and a capacitor having a first end connected to the levelmeasuring electrode and second end of the resistor, and a second endconnected to the ground electrode and a ground terminal of the measuringcircuit.
 28. The method according to claim 27, wherein detecting themeasuring voltage includes detecting a voltage across the first andsecond ends of the capacitor.